Components and subsystems of electronic equipment such as microprocessors, microcontrollers, transformers, filters, semiconductors, transistors, amplifiers, multiplexers, integrated circuits, etc., must operate in restricted temperature ranges. Specifically it is related to spacecraft electronics. This makes thermal control a key matter in the design and operation of a spacecraft with a significant weight, power and cost impact in the overall spacecraft budget.
Spacecraft thermal control relies on the global spacecraft thermal balance: the heat loads must be rejected to deep space that works as a thermal sink. Since no matter links this sink and the spacecraft, this rejection is made by thermal radiation through dedicated radiators installed on the satellite external surfaces.
Spacecraft thermal loads come from the internal spacecraft equipment dissipation and, externally, from the sun and the earth or from the celestial bodies around which the spacecraft orbits. The thermal systems used in spacecrafts must therefore be able to control equipment which operates at a specified range of temperatures and also discontinuously.
At present, known thermal devices for controlling thermal loads in spacecraft are two phase heat transfer loops (HTL) which are also known in engineering practice as capillary driven and mechanically pumped loops or heat loops. The purpose of these devices in a spacecraft is to transfer heat between a heat source (electronic element) and a heat sink (typically, the space). In two phase HTLs heat is transferred through an evaporation-condensation cycle of a working fluid kept inside a hermetically sealed container.
A two phase HTL is filled by working fluid, which is called heat carrier. During nominal operation of the two phase HTL, two phases of this heat carrier, vapor and liquid, are always present in the circuit.
Known two phase HTLs usually comprise at least six elements: an evaporator, a pump, a vapor transport line, a condenser, a liquid transport line and a compensation chamber. Heat applied to the evaporator from electronic equipment is used for phase transformation of working fluid from liquid to vapor. Vapor is moving to the condenser in the vapor transport line. The heat accumulated in the vapor phase is dissipated in the condenser by condensation. Released liquid is transmitted back to the evaporator through the liquid transport line by the pump. The compensation chamber can be installed in different locations of the loop and provides the capability of the loop to operate at different environmental and operational scenarios: to guarantee a sufficient amount of fluid for circulation at cold conditions and to accumulate the excess of liquid due to thermal expansion effect in hot conditions.
Different mechanisms can be used for fluid pumping in the HTL. Capillary driven loops use the capillary suction effect for this purpose and they have a special porous structure, called capillary pump or wick, served for working fluid continuous circulation in the system. The wick is always located in the evaporator. The evaporator is attached to a heat source.
The above-mentioned capillary driven loop technology has found a wide application for thermal control systems in many spacecraft applications, which usually use loops with a single evaporator. However, many applications require thermal control of large thermal contact surface payloads or multiple remotely located heat sources.
Developers of multiple evaporators and multiple condenser designs of capillary driven loops (known in engineering practice as loop heat pipes (LHP), capillary pumped loops (CPL), hybrid two-phase heat loops) intend to create thermal control systems having the following characteristics: optimized functional layout, scalability, expandability, effective heat loads sharing, flexibility in components locations, thermal coupling between separate radiators and minimized mass and volume.
The LHP technology was initially invented in the Soviet Union, and this technology of a heat transfer apparatus is known as per U.S. Pat. No. 4,515,209, for example. Later, a capillary link (secondary wick) between the evaporator and the compensation chamber was introduced to provide liquid supply from the compensation chamber to the evaporator primary wick in zero gravity conditions.
The development and testing of an LHP with two identical evaporators was first performed by the Institute of Thermal Physics (Russian Academy of Sciences) in the mid-80's. Further developments into a multi-evaporator LHP system, as shown for example in USSR Patent 1395927, were carried out using an LHP with two evaporators and two condensers. The two-evaporator LHPs can efficiently operate at symmetrical and non-symmetrical heat load distributions between the evaporators, and at different temperatures of the condenser(s) cooling. However, shutting down the active cooling of one condenser would result in an abrupt decrease in the maximum transport capability of the device.
Every evaporator in the typical LHP system has its own compensation chamber, which can be directly connected to the compensation chambers of other evaporators or can have no direct connection with the compensation chambers of other evaporators in the system. In these devices, evaporators are rigidly connected with each other and are at a relatively close distance from each other.
Despite evident advantages of LHP systems having multiple evaporators designed to operate over a wide temperature range, there exists a limitation on the number of evaporators that can be reasonably used, as each evaporator comprises a compensation chamber. As the minimum operating temperature decreases, the compensation chamber volume increases rapidly when the number of evaporators increases. This leads to a limitation on the number of evaporators that can be used in these systems. It is practically impossible to build an LHP system with more than three evaporators.
Besides, certain problems can also exist with the temperature control in multi-evaporator LHP systems: the key components for the LHP temperature control are the compensation chambers. In a two-evaporator installation, the LHP can operate at the desired temperature in most of the cases, as the LHP responds very well to rapid changes of heat load, sink temperature and set point temperature. However, only one of the compensation chambers has a vapor-liquid two-phase condition during operation regardless of how many are under temperature control.
The heat, which passed by thermal conduction through the capillary pump wall into the central part of the evaporator, in the way opposite to the fluid circulation direction, is called parasitic heat leak. Test results showed that when one of the evaporators has a very low heat load, a sudden vapor generation on the inner surface of the capillary pump was observed, stridently increasing the parasitic heat leak to the compensation chamber which results in a higher operational temperature of the loop. This causes a hysteresis control problem for the loop that is hard to predict or prevent. Also, it was found that situations when the liquid distributes itself among the compensation chambers (trying to occupy the lowest pressure spots) can lead to unstable operation of the system. Furthermore, a problem of controllability for multi-evaporator LHP systems arises when the amount of evaporators and compensation chambers increases.
Therefore, it is possible to conclude that an expandability limitation is the main problem in multi-evaporator LHP systems, as shown in USSR Patent 1395927, such that two evaporators are used or only three evaporators maximum for narrow temperature ranges. A secondary problem presented by these systems too is poor controllability.
Another type of a capillary driven loop is CPL, as for example in documents U.S. Pat. No. 6,626,231 and U.S. Pat. No. 7,118,076, typically comprising one or more evaporators, one or more condensers, transport lines, one remote compensation chamber and a sub cooler. The location of the compensation chamber is the main distinguishing feature between CPL and LHP designs. An LHP compensation chamber (or chambers for LHP multi-evaporator design) is always directly attached to the evaporator but CPL always has one remote compensation chamber (also known as liquid reservoir), separated from the evaporator (or evaporators for CPL multi-evaporator design) by small diameter (2-5 mm) connecting pipe(s). As a rule, in CPL, liquid from the condenser and from the remote compensation chamber flows through the sub cooler before reaching the evaporators. Conversely to LHP the CPL has a reduced ability for self-start up without special preconditioning. Besides, for any CPL, the tolerance for vapor parasitic heat leak is a significant problem of reliable operability of the system. The growing of a vapor bubble on the inner surface of the capillary pump leads to the pump dryout and, finally, to the failure of CPL operation. In case of LHP, the bubble usually migrates into the compensation chamber (as soon as it is closely attached to the evaporator) and condenses in sub cooled liquid which is always presented in the LHP compensation chamber.
Continued improvements have been made to the CPLs in the last decades. The two-port evaporator (one liquid inlet and one vapor exit) initially used in CPLs generally experienced dry-out due to the appearance of vapor in the liquid core during start-up and transient regimes. To prevent vapor from blocking liquid return to the wick structure, a three-port capillary evaporator was introduced in the system connecting the remote reservoir line to the liquid core of the evaporator. This configuration allows vapor to expand along the evaporator core and to migrate into the remote reservoir, instead of accumulating in the evaporator core and interfering with liquid returning from the condenser. Initially, three-port capillary pumps were used as starter pumps, and then like the main functional evaporator design. To prevent vapor from deprimed evaporators to flow upstream and to block liquid return to operating evaporators, a capillary device, known as a capillary isolator, was introduced, located upstream of the evaporator inlet. Back pressure regulators were also installed in many multiple evaporator CPLs to assist start up. These capillary devices, located in the vapor transport line, redirect vapor initially generated at one evaporator to other inoperative (without heat load) evaporators. This action forces liquid from the vapor lines and improves the chances for a successful start up for all evaporators in the system: it is also helping to promote heat load sharing among evaporators, for instance, when an inoperative evaporator acts as a condenser.
The following conclusions summarize the issues related to CPL reliability:                CPL design should never allow bubbles to form in the liquid side of the loop, but it is quite difficult to fully avoid such operational scenario in actual HTLs;        CPL requires a start up evaporator to clear the vapor channels in the main evaporators before heat is applied to them;        reducing the diameter of the CPL evaporator elements leads to many unexpected difficulties: the design with thinner capillary pump walls leads to higher probability of vapor bubble formation inside of the liquid core of the evaporator and as consequence to failure of CPL operation;        
It is known in the state of the art, that in order to improve vapor parasitic heat leak tolerance of evaporators, it is preferable to connect these evaporators in series; in this case the first evaporator in series can develop a sweeping flow for the following evaporators.
Another solution is to have several parallel evaporators connected to the same compensation chamber, located at the evaporating part of the loop, and including special long capillary links between the evaporators and the compensation chamber. This system is known as Free Location LHP CPL, as shown for example in documents U.S. Pat. No. 5,944,092 or Russian Patent 2120592. This system was successfully tested on the ground with a favourable gravitational bias of the evaporators relative to the compensation chamber, making it easy for the capillary links to distribute the fluid to each evaporator. Orientation constraint in gravity field is due to limits imposed by the capillary link. The capillary link connecting the evaporators to the compensation chamber limits the separation distance between the evaporators and the compensation chamber. This limitation is similar to the heat pipes existing in conventional art. Other significant limitations of this design are complexity and integration difficulties which lead to problems of system expandability, scalability and part standardization. All evaporators have to be below or in the same plane with respect to the plane of the compensation chamber. Since the tube connecting each evaporator to the compensation chamber contains a capillary link inside, the tube internal diameter is typically greater than 4 mm, (it is practically impossible to allocate a bendable capillary structure in smaller diameter tubing). Large diameter connecting tubing leads to inflexible system and high requirements for tolerances for integration purposes. In the usual design of a LHP evaporator with a bayonet tube, a capillary link (secondary wick) supplies the primary capillary pump with liquid practically only in transient regimes. However, in this design, the capillary link supplies all amount of liquid that is needed for the evaporator, which leads to significant limitations for rates of change of heat source power or/and heat sink temperature. Other disadvantage of such an approach is the low thermal conductance of evaporators due to the permanent presence of vapor phase in the evaporator core.
An attempt to overcome some of these significant drawbacks led to a so called multi-free LHP CPL known for example per U.S. Pat. No. 5,944,092, where functional evaporators do not have a capillary link to the compensation chamber, only to the liquid line. Limitations of this design are similar to those of ordinary CPLs with starter pumps. Capillary evaporators linked to the liquid line cannot provide a reliable vapor tolerance and, therefore, this design presents the drawback of the necessity of an additional special evaporator with dedicated power source to provide the loop circulation.
Further designs were made developing the so called multi-evaporator hybrid LHP, as known for example in documents U.S. Pat. No. 7,661,464, U.S. Pat. No. 6,889,754, U.S. Pat. No. 7,004,240, U.S. Pat. No. 8,047,268, U.S. Pat. No. 7,549,461, U.S. Pat. No. 8,109,325, U.S. Pat. No. 8,066,055 or U.S. Pat. No. 7,251,889, suggesting that a link between evaporators and compensation chamber could itself be a loop and incorporated this idea in a so called advanced CPL, as an attempt to incorporate both the advantages of a robust LHP and the architectural flexibility of a CPL. This system comprises two relatively independently operated loops, a main loop and an auxiliary loop. The main loop is basically a traditional CPL with the same configuration and operational principles as for CPL, whose function is to transport the waste heat and reject it to a heat sink via the primary condenser. The auxiliary loop is used to remove vapor bubbles from the core of the CPL evaporators and move them to the compensation chamber. The auxiliary loop contains only one LHP-type evaporator with the attached large compensation chamber. The chamber is only one and it is common for all evaporators: the CPL evaporators in the main loop and the LHP evaporator in the auxiliary loop. In addition, the auxiliary loop is also used to ease the start-up process. In this manner, the auxiliary loop functionally replaces the secondary wick of a conventional LHP. The feasibility of this design was however only achieved when the evaporators were connected in series. This means that liquid consequently goes through the evaporators: flow leaving the first evaporator enters the second one, etc.
Initially, the multi-evaporator hybrid LHP included three evaporators, one of which was a standard LHP evaporator directly attached to the common system's compensation chamber, and two traditional three-port CPL evaporators. Tests indicated that the system was not very reliable during power cycling. The sensitivity to power cycle was attributed to the expansion of vapor bubbles in the evaporator core. Heat conduction through the wall of the evaporator capillary pump made it relatively easy to nucleate vapor in the evaporator core. In case of steady state operation, these bubbles were swept from the core of functional evaporators by forward flow of the liquid to the capillary pump. However, as the functional evaporators input power decreased, liquid movement forced by capillary action on the auxiliary evaporator was not enough to efficiently remove all vapor bubbles from the evaporator core to prevent vapor blockage of the capillary pump (dryout) after sudden increase of the evaporator power. On the other hand, sudden power reduction leads to temporary fluid flow break in the condenser until new stable temperature/pressure equilibrium was established in the system. This flow break therefore required a net flow mass displacement from the evaporator and the compensation chamber to the condenser. As a result, nominal forward direction flow was disrupted. During this reversal flow, vapor bubbles could then accumulate or even expand in the evaporator capillary pump core, therefore causing evaporator dry-out and failure of the system.
To improve vapor tolerance, the internal design of the evaporators was modified to include a special phase separation wick, designed to provide better control of the two phases vapor/liquid distribution in the core of the pumps. The design modifications were intended to extend the phase control provided by the secondary wick in the traditional LHP evaporator to the CPL evaporators. Despite general successful results obtained during testing, the operation was verified in relatively limited conditions: mostly in horizontal orientation, evaporators were located close to each other, and therefore with similar hydraulic resistance of lines. Therefore, such configuration was not representative of the conditions of potential spacecraft thermal control application when evaporators and remote reservoir are spatially separated, and the rate of evaporator's response on variations of the input power and heat sink conditions depend on the length of the lines connecting these elements. Therefore, the ability for temperature control was not properly verified.
Also known in the art are hybrid cooling loop technologies, as those shown for example in documents U.S. Pat. No. 6,990,816 and U.S. Pat. No. 6,948,556, which combine the active liquid pumping with the passive capillary liquid management in the wick structure of the evaporator and its liquid/vapor separation. The hybrid cooling loop consists of an evaporator, a condenser, a liquid compensation chamber and a pump as the simplest design. Because of the active amplificatory pumping system, the hybrid loop system could manage different multiple evaporator designs. Despite certain advantages, the necessity of the supplementary loop circulation means can be considered as a drawback because of the active character of critical design components which reduces the reliability and life time of the system.
Another known system developed is the so called advanced LHP which is an LHP with two evaporators: main (functional) and secondary (auxiliary) evaporators, as per document U.S. Pat. No. 6,810,946 B2, for example, incorporating a secondary evaporator to the conventional LHP design. The secondary evaporator is located in a cold-biased environment to ensure that its capillary pump is always primed. Electrical heaters are attached to this evaporator to provide the necessary thermal power for its functioning. With the secondary pump operating, it actively removes the vapor that is accumulated in the compensation chamber by the parasitic heat leaks to the compensation chamber of the main evaporator and to the liquid line. This design considers only a single main evaporator LHP. The main drawback of this approach is the existence of the additional evaporator and its active character. In fact, this solution is needed only for a LHP with not properly designed secondary pump.
Further, an evaporator with attached compensation chamber was proposed to use in a capillary driven loop, known for example per documents U.S. Pat. No. 7,061,446, U.S. Pat. No. 7,268,744 or U.S. Pat. No. 7,841,392. The undivided large capillary wick is used in the evaporator portion and in the compensation chamber. The wick has a greater transverse size in the compensation chamber than in the evaporator portion. There are no means to guarantee vapor tolerance of the evaporators.
Thus, as a summary, it is possible to conclude that the main and the most critical element in a capillary driven loop is the evaporator. The vapor parasitic heat leak intolerance, which can lead to total failure of the system in heat transfer, is the main problem in the development of capillary driven multi-evaporator two phase thermal control systems. Various methods have been proposed and investigated to solve the problem; however, the existing technical solutions still cannot guarantee reliable and stable performance in different actual thermal conditions of spacecraft operation.
The present invention is therefore oriented towards these needs.